Control system methods for networked water heaters

ABSTRACT

Disclosed is a control system for controlling a plurality of fluidly and operably connected water heaters to meet a hot water demand such that overall efficiency is maximized and usage disparity between water heaters is minimized. There is further disclosed a method for detecting a small system demand in said network by adjusting the setting of each flow limiting valve of each water heater. There is still further disclosed a method for enabling seamless addition or removal of a heater in service and heating load distribution to water heaters.

PRIORITY CLAIM AND RELATED APPLICATIONS

This divisional application claims the benefit of priority fromprovisional application U.S. Ser. No. 61/149,418 filed Feb. 3, 2009 andnon-provisional application U.S. Ser. No. 12/699,487 filed Feb. 3, 2010.Each of these applications is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to water heaters, and morespecifically, to methods for a control system used with fluidly andfunctionally connected water heaters.

2. Background Art

The art of using water heaters in cascaded fashion to meet large waterheating loads is not new. In a cascade system, a plurality of waterheaters is used to participate in sharing water heating load to meet ademand. Typically, in a large commercial building, apartment complex,hotel or laundromat, the demand for hot water can range from zero to avery large demand in an instant. Therefore, a system capable ofproviding a demand in real time ranging from very small to very large isneeded.

A single commercial or residential hot water heater is incapable ofproviding such a wide ranging demand in real time. Another drawback ofusing a single unit under such circumstance is that it provides a singlepoint of failure. When a single water heater is removed for repair ormaintenance, the entire building would be without hot water. Otherdrawbacks of using a single water heater include the excessive physicalsize and inefficient heating associated with excessive physical size.One solution commonly used in solving the drawbacks associated withusing a single heater for a severely varying and large demand is toleverage the heating capacity of multiple water heaters. As such,multiple water heater units may be cascaded to be fluidly andfunctionally connected to form a network of water heaters so that onewater heater can be turned on to service a small demand while multipleunits can be turned on simultaneously to service a sudden change to alarge demand. Furthermore, a cascaded system involving multiple waterheaters affords failure redundancy not available in a single waterheater system. One or multiple units may be removed for service withoutinterrupting the operation of remaining water heaters in the network.

In a conventional cascade system, the last water heater turned on is themodulating boiler while the capacity of other previously selected andturned on water heaters is pegged at their maximum output. As such, thelast water heater turned on may experience excessive cycling on and offif a requested demand falls within a dead band. Various control schemeshave been devised to provide for those situations where the overall heatdemand for the system of a plurality of water heaters falls within azone lying between the maximum heat output of a water heater and the sumof the maximum heat output of this water heater and the minimum heatoutput of the next adjacent water heater. This zone, which may bereferred to as a dead band or dead zone, presents unique operationalproblems because the next adjacent boiler cannot modulate within thatrange.

Published US patent application 2008/0216771 entitled “CONTROL SYSTEMFOR MODULATING WATER HEATER” discloses a control system which minimizesthe cycling on and off of such next adjacent boiler if the overalldemand falls in a dead band. The control system is claimed to beparticularly suited for use with a plurality of modulating waterheaters, which may be boilers, arranged for control in a cascadesequence where a first boiler is brought online at its firing point andis then continuously modulated up to its maximum output, and then, thefirst boiler is maintained at its constant output while firing a secondboiler which is then modulated from its firing point up to its maximumoutput as the overall heat demand on the system increases. In a similarmanner, each boiler is brought up to its maximum output before the nextadjacent boiler is fired, and all previously fired boilers aremaintained at maximum output with the modulation for the system comingfrom modulation of the last fired boiler. While heater cycling isminimized, the '771 application falls short of addressing the issue ofdistributing flow to conserve energy.

In conventional cascade water heater systems, there often exists asignificant disparity in usage between water heaters in a network. Theorder in which water heaters are turned on is fixed. A small demandcauses a first water heater to turn on. As demand increases, more waterheaters are turned on. As a result, the water heaters arranged to turnon first experience significantly higher accumulated usage than others,especially ones serving low demands. Water heaters experiencing higheraccumulated usage require more regular preventative or unscheduledmaintenance while others are underutilized. One attempt to solve such aproblem is evidenced in water heaters marketed under the trade name“Eternal Advanced Hybrid Water Heating” by Grand Hall Enterprise, Ltd.The operator's manual labeled 157110293 and dated Jul. 4, 2009introduces a host and sub concept in which a host unit is selected asthe first unit to fire when demand for hot water is detected and itcontrol multiple sub units. According to “Specifications and Features”(page 3) and “MCU Operational Sequence Flow Chart “(page 8) sections ofthis operator's manual, the designation of a water heater controller asthe host is changed every 24 hours in order to distribute wear and tearacross all units in a networked system.

Published US patent application 2008/0216771 further discloses a schemein which each boiler includes a controller and may serve as a leadboiler and its controller as the master controller. The role of leadboiler is periodically rotated between each of the boilers in the systemso as to substantially equalize the number of operating hoursexperienced by each boiler. The practice of using operating hours aloneas a measure to estimate a water heater's remaining life is fraught withuncertainties since there are other significant factors affecting thewater heater's remaining life. In use, a water heater delivers an amountof hot water at a temperature over a period of time. Given a fixednumber of operating hours, the damage done to a water heater used todeliver water at 140 degrees Fahrenheit is substantially different thanthe damage done to a water heater used to deliver water at 102 degreesFahrenheit. Based on this premise, the applicants believe that thereneeds to be an improved or more accurate method of estimating remaininglife to efficiently control water heaters in a networked or cascadedsystem.

SUMMARY OF THE INVENTION

The present device overcomes the shortcomings of the prior art byproviding one or more structures and methods for controlling waterheaters in a networked or cascade system. In accordance with theteachings of the present invention, there is provided a method forcontrolling a plurality of fluidly and operably connected water heatersin a network to meet a system demand. The method uses overall efficiencyand usage history as two primary factors in determining the heating loada water heater is required to provide in order to meet the systemdemand. The method comprises the steps of providing a valuecorresponding to relative remaining life of each water heater of thenetwork and a value corresponding to minimum output of each water heaterof the network, obtaining a number of participating water heatersrequired to service the system demand, obtaining an average heatingload, selecting participating water heaters and setting and activatingeach of the participating water heaters at the average heating load toprovide a total load meeting the system demand.

One challenge encountered with controlling a plurality of fluidly andoperably connected water heaters in a network is to provide the abilityto detect a small system flow. In accordance with the teachings of thepresent invention, there is provided a method for detecting a smallsystem flow. The method takes advantage of a flow limiting valve, a flowsensor, a value corresponding to a predetermined potential system demandand a value corresponding to the maximum output of a water heater in thenetwork. The method comprises the steps of obtaining a number ofparticipating water heaters required to service the predeterminedpotential system demand, obtaining an average flow limiting valvesetting, selecting participating water heaters and setting andactivating the flow limiting valve of each of the participating waterheaters at the average flow limiting valve setting.

In one aspect of the invention, a unique control scheme is provided toenable the processes of seamlessly adding a water heater to or removinga water heater from and assigning a heating load to a water heater in afluidly and operably connected water heater network, wherein each waterheater of the network has a controller which communicates with otherwater heater controllers within the network via a communication bus andeach controller maintains a list of usage planning data. The methodcomprises the steps of supplying power to each water heater of thenetwork, configuring the controller of each water heater to broadcast amessage to a communication bus of the network, wherein the messageincludes an ID (identification) and usage planning data, configuring thecontroller of each water heater to listen to and receive messages fromother controllers on the communication bus, reconciling the list ofusage planning and sorting the list based on a predetermined key toproduce a sort result, executing a function based on the sort result andconfiguring the controller of the water heater to update its usageplanning data.

Accordingly, a feature and advantage of the present device is itsability to provide a cascade water heater control system that maximizesthermal efficiency while minimizing disparity in wear across all waterheaters in the system.

It is another object of the present invention to provide a cascade waterheater control system that minimizes operating cost while minimizingdisparity in wear across all water heaters in the system.

It is yet another object of the present invention to provide a cascadewater heater control system capable of detecting a small flow.

It is yet another object of the present invention to provide a cascadewater heater control system that is plug and play.

Whereas there may be many embodiments of the present invention, eachembodiment may meet one or more of the foregoing recited objects in anycombination. It is not intended that each embodiment will necessarilymeet each objective. Thus, having broadly outlined the more importantfeatures of the present invention in order that the detailed descriptionthereof may be better understood, and that the present contribution tothe art may be better appreciated, there are, of course, additionalfeatures of the present invention that will be described herein and willform a part of the subject matter of this specification and claims. Thepresent invention is capable of other embodiments and of being practicedand carried out in various ways. Also it is to be understood that thephraseology and terminology employed herein are for the purpose ofdescription and should not be regarded as limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained, a more particular descriptionof the invention briefly described above will be rendered by referenceto specific embodiments thereof which are illustrated in the appendeddrawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be considered tobe limiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a block diagram illustrating a water heating system comprisinga plurality of water heaters controlled using the present controlsystem.

FIG. 2 is a flow diagram illustrating a novel method of the presentcontrol system used to assign heating load to all water heaters in asystem including a plurality of water heaters.

FIG. 3 is a flow diagram illustrating a novel method of the presentcontrol system used to detect a small hot water demand in a systemincluding a plurality of water heaters.

FIG. 4 is a flow diagram illustrating a novel method of the presentcontrol system used to enable seamless addition or removal of a heaterin service and heating load distribution to water heaters.

The drawings are not to scale, in fact, some aspects have beenemphasized for a better illustration and UNDERSTANDING OF THE WRITTENDESCRIPTION.

PARTS LIST

2—first water heater

4—second water heater

6—third water heater

8—boiler or heating means

10—flow limiting valve

11—pump

12—water heater controller

14—point of demand

15—system demand

16—system demand requested at point or points of demand

18—not used

20—step of obtaining number of participating water heaters

22—step of obtaining average heating load

24—step of selecting participating water heaters based on relativeremaining life

26—step of setting and activating participating water heaters to averageheating load

28—list of relative remaining life of all water heaters in network

30—not used

32—step of obtaining number of participating water heater valvessufficient to enable potential system flow

34—step of obtaining average valve setting for an individual waterheater valve

36—step of selecting participating water heater valves based on relativeremaining life

38—step of setting and activating participating water heaters to averageindividual valve setting

40—step of supplying power to a controller

42—step of broadcasting internal ID and usage planning data

44—step of listening to or receiving external ID and usage planning data

46—step of updating list

48—step of removing old data from list

50—step of sorting remaining data in list based on a key

52—step of executing a function based on sort result

54—decision to execute additional function

56—step of updating internal usage planning data

TI—input water temperature

TO—output water temperature

QT—total flow

Q1, Q2, Q3—individual flow through water heaters A, B and C

Particular Advantages of the Invention

In accordance with the present invention, a novel control method usesefficiency and usage history as two primary factors in determining theheating load a water heater is required to provide in order to meet asystem demand. Thus, the usage is distributed amongst all water heatersconnected in a network in order to minimize usage variation betweenindividual units and maximize efficiency.

The practice of using operating hours alone as a measure to estimate awater heater's remaining life is fraught with uncertainties since thereare other significant factors affecting the water heater's remaininglife. In accordance with the present invention, the concept ofnormalized operating hours is used, where normalized operating hoursrepresent expended energy, thermal cycle count, blower speed, flow rate,outlet-inlet water temperature difference, and the like.

In a fluidly connected network of a plurality of water heaters, the flowexperienced in each water heater of the network is lower than the flowexperienced in a system consisting of a single water heater if the flowthrough each water heater of the network is unrestricted. The flow rateexperienced in a water heater in a network is a fraction of the totalflow of a system. Therefore, a demand that is detectable in a singlewater heater system may not be detected by a flow sensor associated witha water heater in a network of water heaters. In accordance with thepresent invention, there is provided an ability to detect a small systemdemand in a plurality of fluidly connected water heaters.

In accordance with the present invention, a true masterless controlscheme is provided. The control scheme does not require physical setupof an identification number during installation nor does it require amaster-slave designation which unnecessarily complicates the functionalrelationship of one water heater with other water heaters in a network.The ability to add or remove a water heater at will without disruptingthe existing service is provided. Continuity of service is provided evenwhen there is one or more water heaters that are non-functional or whenone or more water heaters have been removed for service or repair. Whensuch an event occurs, a water demand is met by heating load distributionto the remaining functional water heaters. When necessary, additionalwater heaters may be added without disrupting existing water heatingoperation.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As a general overview, and with reference to FIG. 2 in particular, thereis provided a method for controlling a plurality of fluidly and operablyconnected water heaters in a network to meet a system demand. The methodcomprises the steps of: (1) providing a value corresponding to relativeremaining life of each water heater of the network and a valuecorresponding to minimum output of each water heater of the network 16,(2) obtaining a number of participating water heaters required toservice the system demand 20, (3) obtaining an average heating load 22,(4) selecting participating water heaters 24 and (5) setting andactivating each of the participating water heaters at the averageheating load 26 to provide a total load meeting the system demand. Aswill be described in greater detail with respect to the particular stepsof the method, the method uses efficiency and usage history as twoprimary factors in determining the heating load a water heater isrequired to provide in order to meet the system demand. Efficiency maycontemplate thermal efficiency, least cost option or both.

The novel control method is illustrated and described with reference toFIG. 2, however, for a better understanding of the novel control method,it is instructive to describe a networked water heater system in whichthe control method may be used. FIG. 1 is a block diagram illustrating awater heating system comprising a plurality of water heaters 2, 4, 6controlled using a novel control system (described in greater detailbelow with reference to FIGS. 2-4). In the example illustrated in FIG.1, three water heaters 2, 4, 6 (also designated A, B, C on FIG. 1) areconnected in parallel configuration such that a system demand 15 iscooperatively met by the flows Q1, Q2 and Q3 of water heaters 2, 4 and 6respectively. A total of six points of demand D1, D2, D3, D4, D5 and D6are shown in this figure, which cumulatively form the system demand 15.However, it is to be understood that the example presented herein is forillustrative purposes only. Any number of water heaters, flow limitingvalves and pumps may be controlled using the present control system andany number of points of demand 14 may be used, provided that thecombined heating capacity of the system is sized appropriately to meetdemand.

Each of the water heaters 2, 4, 6 comprises a flow limiting valve 10, apump 11, at least one heating means 8 and a controller 12. The fluidflow through each flow limiting valve 10 can be altered from zero flowto a flow corresponding to a fully open (maximum flow) valve setting.The heating means 8 is commonly a burner, a combined blower and burnerunit or an immersive electric element, however the control system is notso limited. Any known or developed means of heating hot water may besuitably adapted to the present system. There are numerous ways toprovide hot water. Residential and commercial applications typically usegas or electric water heaters or a combination thereof. A gas waterheater typically uses a burner to generate heat and a heat exchanger totransfer the generated heat to the water supply demanded by a user.Traditionally, a large reserve tank is used to hold a large volume ofheated water in anticipation of a hot water request. If a request is notimminent, heat energy is unnecessarily lost to the surroundings andwasted. Recently, tankless water heaters have gained popularity due toconcerns of high energy costs and depletion of energy sources.

In a tankless water heater, a demand of hot water is met by nearinstantaneous heating of water to the user. Little is wasted since wateris not heated long before it is used. A tankless water heater comes in avariety of configurations. A high efficiency model typically comes witha blower with or without speed control. The use of an electric blower iscommon in heaters involving inverted burners where hot combustion gasesare forced down against gravity through a heat exchanger coil.

An electric water heater, on the other hand, typically comprises animmersion heating coil which comes in direct contact with a body ofwater to be heated. Even though the ensuing discussion focuses on acontrol system designed for tankless water heaters, each comprising avariable speed blower, a flow limiting valve and a pump, it is to beunderstood that the present inventive concepts are applicable to waterheaters of other modes of heating or having equivalent controlinstruments.

For the purposes of the following examples, a combined blower and burnerunit will be used. In use, when the system demand 15 becomes non-zero, acold water flow with a flow rate of QT and temperature of T1 isestablished upstream of the water heaters 2, 4, 6. This total cold waterflow rate QT is made up of individual flows with flow rate of Q1, Q2 andQ3 which are fed through water heaters 2, 4, 6 respectively, to beheated to temperature TO. Flow rates Q1, Q2 or Q3 are sized based on theheating load assigned to water heaters 2, 4, 6 by their respectivecontrollers 12. The method by which each water heater is assigned aheating load is the subject of the present invention. The heating loadassigned to each heater directly corresponds to the flow allowed to flowthrough each heater.

In one aspect of this embodiment and referring again to FIG. 1, flowrate control is further aided with the use of a pump 11 connected inlinewith a flow limiting valve 10. Typically, a tankless water heater isequipped with a pump that cycles water internally within the internalplumbing of the water heater to deliver water at a desired temperature.Where the pump is not used for internal recirculation, the method mayoptionally and additionally include the step of activating the pump toincrease flow rate. Where opening the flow limiting valve alone isinsufficient in increasing a flow to a desired flow rate quickly due tolimited water pressure, the method may optionally and additionallyinclude the step of activating the pump to increase flow rate toexpediently bring the flow rate to the desired level.

FIG. 2 is a flow diagram illustrating a novel method of the presentcontrol system used to assign load to all water heaters in a system. Thesystem has a plurality of water heaters that are interconnectedoperably, fluidly and electronically in a network. The method usesoverall efficiency and remaining life as keys to assigning load to waterheaters of the network. Heating load is assigned based on a distributionwhich maximizes overall efficiency and minimizes disparity in remaininglife across all water heaters in a network.

The present invention uses a novel true masterless control scheme,wherein each controller of the network is responsible for determiningthe actions the water heater has to take to fulfill a system demand.Although a master-slave control system, which is commonly used in priorart cascade water heating systems may be used to benefit from thepresent novel method of assigning heating load to each water heater, thebenefits of using a true masterless control scheme will become apparentin discussions pertaining to FIG. 4.

In one aspect, thermal efficiency is the amount of thermal energy outputversus thermal energy input. In systems where the primary energyconsuming equipment is a gas burner, thermal efficiency is a suitablerepresentation of the overall efficiency. Thermal energy output in aburner unit is a measure of the work done to increase the temperature ofan amount of water by a certain number of degrees. Thermal energy inputin a burner unit is a measure of the heat content of a fuel that isconsumed in order to produce a thermal energy output. In any burnerbased systems, there exists an operating point that corresponds tocondition where the highest thermal efficiency is achieved. It has beendiscovered that thermal efficiency is inversely proportional operatingcapacity, i.e., the flow rate. As the flow rate increases, the thermalefficiency of a burner system drops. In order to achieve maximum thermalefficiency, the smallest flow rate is desired. However, a typical waterheater is capable of detecting a flow only if its flow rate rises abovecertain threshold. Therefore, there exists a minimum flow rate or outputrequirement which must be met before the flow rate of a water heatingsystem can be meaningfully modulated. The heat load related to thisminimum flow rate is termed minimum heat load hereinafter.

In a second aspect of this embodiment, overall efficiency is the leastcost option. In a heating system comprising various energy consumingequipment components, energy usage is not limited to a single mode. Forinstance, a combined gas burner and blower system consumes both fuelenergy and electric energy. As another example, a combined gas burnerand immersive electric element system also uses both fuel and electricenergy. Furthermore, it is well known that the cost per unit electricalenergy can differ tremendously from the cost per unit thermal energy.The per unit cost for each mode of energy can also change dailydepending on its market value at a given time. Therefore, the practiceof optimizing heating load distribution based merely on thermalefficiency alone may not meet consumer objectives. In this second aspectof the embodiment, the control method utilizes a heating distributionmeans based on least operating cost. As an example, in a cascade systemhaving two water heaters, a system demand which can be met using onewater heater alone may be equally distributed to both water heaters. Inaccordance with the least operating cost strategy, the decision to useone or both water heaters is based on the expected total operating costof using one as compared to two water heaters to meet the same systemdemand.

Remaining life shall be defined as a measure indicating the amount ofuseful life a water heater has until a repair, maintenance orreplacement is required, or anticipated to be required based onstatistical data. Remaining life is inversely proportional to the amountof “damage” which has been inflicted upon it. Water heater damage isaffected by various factors related to the normal operation, such as,for example, the expended energy, thermal cycle count, deltaT, flow rateand blower speed associated with each water heater. The expended energyis defined as the cumulative energy (for example, thermal andelectrical) consumed in operating a water heater. The thermal cyclecount is defined as the number of events where the flow rate through awater heater changes by 1.5 gallons per minute (“gpm”). deltaT isdefined as the difference between heater outlet and inlet temperature.To illustrate, damage at any given time is calculated as the addition ofscaled sum of energy expended raised to the power X, scaled sum ofthermal cycle count raised to the power Y and scaled sum of blower speedmultiplied by flow rate and deltaT raised to the power Z over time,where X, Y, Z, A, B, C are factors specific to a water heater or asystem having a plurality of water heaters.

Damage=A*(sum of(energy expended)̂X over time)+B*(sum of(thermal cyclecount)̂Y over time)+C*(sum of(blower speed*flow rate*deltaT)̂Z)over time)

Referring again to FIGS. 1 and 2, a system demand 15 is first requestedas shown in step 16 at one or more points of demand 14. Upon detecting asystem demand, each controller 12 executes a series of steps todetermine the amount of heating load it needs to provide to achieve bothmaximum overall efficiency and optimal usage distribution.

In Step 20, the number of participating water heaters required to meetthe system demand 15 detected in step 16 is determined. The number ofparticipating water heaters is obtained by dividing the system demand 15by the minimum output of each water heater. For instance, if the systemdemand 15 is 10 gpm and the minimum output of each of the water heatersis 1 gpm, the number of participating water heaters would be ten ifthere are ten or more available water heaters in the network. Using theoverall efficiency method based on thermal efficiency, the demand wouldbe met by using as many water heaters as possible. If there are fiveavailable water heaters, the number of participating water heaters wouldbe limited to five. However, if the system demand 15 is 3 gpm and thereare five available water heaters, the number of participating waterheaters would then be three since each of the water heaters is capableof providing a minimum heating load of 1 gpm to meet a total of 3 gpmsystem demand. In such a case where there is at least one water heaterthat is not required to be turned on, it becomes a “reserve waterheater.” When a reserve water heater exists, various measures may betaken in anticipation of an increase in demand: starting internal and/orexternal recirculation circuit of a tankless water heater to minimize adelay in delivering hot water to a demand point.

In step 20, if the least cost option is selected, the system demand willbe satisfied by using a group of participating water heaters such thatthe system would incur the lowest operating cost. In one embodiment, thecontroller of each water heater calculates the expected total operatingcost of operating one or more water heaters to meet a system demandbased on the operating parameters of the water heater it controls. Inanother embodiment, the controller of each water heater calculates theexpected total operating cost of operating external water heaters basedon operating parameters received from external water heaters.

From the determined number of participating water heaters, an averageheating load is determined. Step 22 involves calculating the averageheating load of a participating water heater. The average heating loadis obtained by dividing the system demand 15 by the number ofparticipating water heaters.

Block 28 represents a list of relative remaining life of all waterheaters retrieved from a memory location functionally connected to thecontroller of a water heater. In Step 24, participating water heatersare selected from all water heaters in the network. This step isachieved by selecting the required number of participating water heatersfrom those water heaters having the longest remaining life. Forinstance, assume the number of required participating water heaters tobe three. There are five water heaters 1, 2, 3, 4, 5 having remaininglife values of L1, L2, L3, L4 and L5 respectively:

-   -   L1 is less than L2,    -   L2 is less than L3,    -   L3 is less than L4, and    -   L4 is less than L5.

In this case, then L3, L4 and L5 would be selected as participatingwater heaters in Step 24 since they possess longer remaining life ascompared to L1 and L2.

In Step 26, each water heater's controller identifies whether it is oneof the selected participating water heaters by comparing its ID to theID associated with each participating water heater. If a positiveidentification is returned, the water heater is turned on or activatedto meet the system demand by supplying the average heating load to thesystem.

In accordance with the present invention, the control system uses a truemasterless control scheme which does not require physical setup of anidentification number during installation nor does it require amaster-slave designation which unnecessarily complicates the functionalrelationship of one water heater with other water heaters in a network.The concept of rotating the role of lead water heater in a multiplewater heater setup is not new. Published US patent application20080216771 discloses such a scheme in which each boiler includes acontroller and may serve as a lead boiler and its controller as themaster controller. The role of lead boiler is periodically rotatedbetween each of the boilers in the system so as to substantiallyequalize the number of operating hours experienced by each boiler. Asubstantially similar strategy is employed by Grand Hall Enterprise,Ltd. in its “Eternal Advanced Hybrid Water Heating.” The operator'smanual labeled 157110293 and dated Jul. 4, 2009 introduces a host andsub concept in which a host unit is selected as the first unit to firewhen demand for hot water is detected and it control multiple sub units.In this setup, the designation of a water heater controller as the hostis changed every 24 hours in order to distribute wear and tear acrossall units in a networked system. Each controller of this setup isequipped with a dipswitch which must be physically or correctly setprior to use in order to distinguish one controller from another. Thepresent novel control method improves upon the prior art by removing theneed for such a manual step which is not only time consuming but alsocostly due to the additional hardware required and prone to installationerror.

A true masterless control scheme of the present invention enables theaddition of a water heater simply by connecting the water heater fluidlyto all existing water heaters in the network and connecting the waterheater's controller to a communication bus shared by all existing waterheater controllers in the network. The true masterless control schemefurther enables the removal of a water heater from service in thenetwork simply by disconnecting the water heater fluidly from thenetwork and disconnecting the water heater's controller from thecommunication bus shared by all existing water heater controllers in thenetwork. Each controller of a water heater in a true masterless controlscheme is responsible for determining the actions the water heater hasto take to fulfill a system demand, thereby simplifying the setup of awater heater network and minimizing the potential for setup errors.

FIG. 3 is a flow diagram illustrating a novel control system method usedto detect a small hot water demand in a system with a plurality of waterheaters. Typically, a water heater is equipped with a flow sensorcapable of detecting a flow rate above a minimum threshold. In a fluidlyconnected network of a plurality of water heaters, the flow experiencedin each water heater of the network is lower than the flow experiencedin a system consisting of a single water heater if the flow through eachwater heater of the network is unrestricted. The flow rate experiencedin a water heater in a network is a fraction of total flow of a system.Therefore, a demand that is detectable in a single water heater systemmay not be detected by in a network of water heaters.

The present novel control method enables fluidly connected water heatersto detect a demand having a flow rate equal or greater than the minimumdetectable threshold of a water heater, in particular water heatershaving a flow limiting means, such as that provided by a flow limitingvalve. When the water heaters of a network are first turned on or when asystem demand has ceased or when a demand has dropped below apredetermined limit, a control procedure is executed to prepare for thedetection of the next small system flow.

In Step 32, the number of participating water heaters required to meet apotential system flow is determined. A potential system flow is definedas a typical starting flow that is predetermined based on an expectedusage habit of the water heating system. The number of participatingwater heaters is determined by dividing the potential system flow by themaximum output of a water heater in the network. For instance, if thepotential system flow is 9 gpm and the maximum output of a water heaterin the network is 5 gpm, the number of participating water heater isthen determined to be two.

In Step 34, an average valve setting for each participating water heateris determined. The average valve setting is obtained by dividing thepotential system flow by the number of participating water heaters.Continuing on the foregoing example, the average valve setting isdetermined to be 4.5 gpm.

In step 36, participating water heaters are selected from the network.This step is achieved by selecting a number of water heaters having thelongest remaining life with reference to a list of relative remaininglife of all water heaters retrieved from a memory location functionallyconnected to the controller of a water heater (Block 28). Uponidentifying the flow limiting valves whose valve setting needs to beadjusted, step 38 proceeds to set and activate the valve setting of theidentified flow limiting valves to correspond to 4.5 gpm each.

FIG. 4 is a flow diagram illustrating a present novel control systemmethod used to enable seamless addition or removal of a heater inservice and heating load distribution to water heaters. A plurality ofwater heaters is fluidly connected and the controller of each of theplurality of water heaters is operably connected in a network. Eachcontroller is preferably equipped with a communication means common toall controllers on the network that communicates via a common bus. Anexemplary protocol commonly used in the industry is the Modbus protocolcommunicated over a serial EIA-485 physical layer. Other communicationprotocols supporting hostless communication, such as CAN or Ethernet andtheir supporting hardware may also be suitably adapted to perform theintended function. The communication bus is established via hard-wireconnection or wireless means.

The ensuing description will be presented from the perspective of acontrol algorithm running in a controller. When electrical power issupplied to a controller (step 40), a routine is started where thecontroller starts making one or more broadcasts to the communicationbus. In Step 42, a message comprising an internal ID (identification)code used to identify the controller from which the message originatesand a set of usage planning data is broadcasted. In one embodiment, theID is a factory-set serial number unique to a controller. The messagecomprises pertinent information which all the controllers in the networkrequire to determine heating load distribution. In one embodiment, ausage planning data set comprises a normalized relative remaining lifevalue of the broadcasting controller.

In Step 44, the communication bus is listened to and broadcast messagesfrom external controllers are received. Identical to the internalmessage format, the external messages also comprise IDs and usageplanning data sets. Upon receiving external messages, the controllerparses pertinent information and inserts such information into a list.In one embodiment, the ID and normalized relative remaining life valueare harvested from each message and put on such list.

In Step 46, the list is reconciled by removing expired data points fromthe list. A data point harvested from a broadcast message received froman external source and inserted into the list is retained for apredetermined amount of time. Upon the expiration of this predeterminedamount of time, the data is removed from the list (step 48). However, ifa new message is received from a water heater whose ID exists in thelist, the current data point will be replaced by the data point parsedfrom the new message. As such, a water heater which has just becomeunavailable will be removed from the list while a newly added waterheater to the network or an existing available water heater will remainand acknowledge its availability by broadcasting messages.

Upon finishing reconciliation of the list's data points, the controllerproceeds to sort the remaining data points (step 50) in the list basedon a preprogrammed key. In one embodiment, the preprogrammed key is therelative remaining life. When sorting is complete, the list is orderedsuch that the data points are arranged in an ascending or descendingorder based on the relative remaining life. The controller then proceedsto execute a pre-designated function based on the sort result (step 52).In one embodiment, the pre-designated function is responsible forassigning a heating load to a water heater. The controller furtherdetermines whether another function is pending (step 54). If a pendingfunction exists, the sorting step 50 is repeated with a preprogrammedkey. If no additional functions are pending, the controller proceeds toupdate the usage data that belongs to the water heater it controls (step56). The sequence is then repeated where the broadcast step 42 is againexecuted.

Thus it is seen that the methods of the present invention readilyachieve the ends and advantages mentioned as well as those inherenttherein. As will be readily appreciated by those skilled in the art, thepresent control methods are capable of other embodiments and of beingpracticed and carried out in various ways within the spirit ofApplicant's inventive concept.

1. A method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand, comprising steps of: providing a value corresponding to relative remaining life of each said water heater, a value corresponding to a minimum output of each said water heater, a flow limiting valve for each said water heater and a pump for each said water heater; obtaining a number of participating water heaters required to service said system demand by dividing said system demand by said value corresponding to minimum output; obtaining an average heating load by dividing said system demand by said number of participating water heaters in said network; selecting participating water heaters by identifying said number of participating water heaters having the longest relative remaining life; and setting and activating each of said selected participating water heaters at said average heating load to provide a total load meeting said system demand.
 2. The method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand of claim 1, wherein said step of obtaining a number of participating water heaters comprises dividing said system demand by said value corresponding to minimum output.
 3. The method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand of claim 1, wherein said step of obtaining a number of participating water heaters comprises estimating the total cost of operating different quantities of water heaters of said plurality of water heaters where the heating load is substantially equally distributed over said different quantities of water heaters of said plurality of water heaters and selecting the number of participating water heaters that yields a lowest cost.
 4. The method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand of claim 1, wherein said value corresponding to relative remaining life comprises normalized factors of expended energy, thermal cycle count, blower speed, flow rate and outlet-inlet water temperature difference of a water heater.
 5. The method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand of claim 1, wherein each of said plurality of water heaters has a controller, wherein distribution of a heating load to each of said plurality of water heaters is determined by its controller.
 6. The method for controlling a plurality of fluidly and operably connected water heaters in a network to meet a system demand of claim 1, wherein each of said water heaters has a controller, wherein the heating load corresponds to a flow which is achieved by controlling concurrently said flow limiting valve of each water heater and said pump of each water heater.
 7. A method for detecting a small flow in a plurality of fluidly and operably connected water heaters in a network, each of said plurality of water heaters has a flow limiting valve having a flow range of from zero to a non-zero value, a flow sensor having a minimum detectable flow, a value corresponding to a predetermined potential system demand, a value corresponding to the maximum output of a water heater in said network, comprising steps of: obtaining a number of participating water heaters required to service said predetermined potential system demand by dividing said value corresponding to said predetermined potential system demand by said value corresponding to the maximum output of a water heater in said network; obtaining an average flow limiting valve setting by dividing said predetermined potential system demand by said number of participating water heaters in said network; selecting participating water heaters by identifying said number of participating water heaters having the longest relative remaining life; and setting and activating said flow limiting valve of each of said selected participating water heaters at said average flow limiting valve setting. 